Ok, but who owns them? If the investors are some hedge/mutual funds that invested significant (more than, say, 5%) share of their total capital in Uber or Lyft then you probably can short those funds instead.
sure you can, but lyft is very likely already written off. meaning you can short the fund (probably not because those funds are private, as well) but the lyft losses are already priced into their "stock price".
edit: maybe its more obvious in a different industry: assuming a hurricane wrecks florida. now you want to profit from shorting that event, so you short disaster insurance providers because you know they will have to pay for the damages. but the public market was faster. the losses are already priced into the stock, although those losses havent been realized yet. you can short it all you want, but you will be "too late" with your "unique insight".
What you are saying might be right in principle: markets usually factors in risk in current prices. However, it seems to me that technological startups, like Uber, but also Airbnb, Twitter, etc. are market's blind spot.
There is a lot of hype in media, everybody is seeking new unicorn to invest in, so it might be the case that all the funds managers are still in denial about the true worth of technology companies.
It wouldn't be uprecedented, in fact current market situation looks a lot like just before dot-com bubble bust: 8 years of bull market, and a lot of money invested in companies without viable business model, like Uber.
the idea that uber is not viable is a bit of a stretch. it is certainly a good service used by many millions of people.
you seem to lack a solid grasp of economics. the reason uber is "losing money" is because it is competing in an oligopoly and tries to get rid of its competition. the reason it is being funded to "lose" billions of dollars every year is because it is clearly valuable, not because some "hedge fund idiots" dont know what to do with their money.
once uber turns into a quasi monopoly, it will be able to fix its pricing structure. if it goes too far, it will be regulated. uber is already more of a public utility than "just a company".
calling something that millions of people love "not viable" has never worked. and never will. uber and airbnb are solving the major problem of "you have to own a house and a car". thats a major achievement and costs a lot of money to establish. especially due to regulation trying to prevent people from becoming more mobile and less enslaved.